1. Field of the Invention
The invention relates to a method of reactive coating of substrates in a sputtering apparatus operating on the magnetron principle. For instance, coatings having optical properties or corrosion protection coatings may be fabricated by this method.
In reactive coating, an electrically conductive target, preferably Al, Ti, In, C, is sputtered. The introduction of reactive gases such as O.sub.2, N.sub.2, H.sub.2, C.sub.x H.sub.y and others into the discharge chamber leads to the formation of such compounds as Al.sub.2 O.sub.3, SiO.sub.2, TiN, for instance.
By guiding the discharge plasma in an integrally enclosed magnetic tunnel formed by arcuate curved magnetic field lines in front of the target, the density of the plasma and the sputtering rate are increased. The reactive magnetron discharge is determined, among others, by parameters depending upon the target material and upon the reaction gas. These parameters are the discharge current intensity and the discharge voltage which essentially define the operating point of the process. On account of the reactive gas component the magnetron discharge is provided with an additional degree of freedom. With a great many target reactive gas combinations instabilities occur at least in portions of the operating range. The discharge moves to a completely different operating point having different parameters and, therefore, other coating properties.
2. The Prior Art
For stabilizing the reactive magnetron discharge, it is known to select the partial pressures of the inert gas and the reactive gas such that the discharge current or the discharge voltage remains constant at a changing reactive gas pressure and to utilize the dependence of the discharge current or discharge voltage from the partial pressure of the reactive gas for controlling the reactive gas (DD 146,306). Furthermore, an analogous control process is known by which the operating point at the associated reactive gas flow is calibrated and maintained constant by metering of the introduced reactive gas (DE 4,106,513 A1). These processes suffer from the disadvantage of being suitable only for a relatively brief stabilization for a fraction of the target use time, since during the target use time the effective magnetic field strength and, hence, the discharge parameters and the reaction kinetics change as the depth of erosion increases, which leads to changed properties of the deposited layers. For this reason, the operating point requires constant adjustment in response to measured layer characteristics. Furthermore, these methods are of use only in connection with magnetron sputtering sources having but one discharge plasma. Furthermore, a reverse sputtering zone of increasing width is formed at the target edges as a result of the focussing effect of the erosion moat. Particularly with insulating layers such as Al.sub.2 O.sub.3 or SiO.sub.2, these reverse sputtering zones lead to arc discharges and, hence, to interfering particles in the deposited layer. A further significant disadvantage of the currently known solutions is that they do not allow the realization of a reactive sputtering process by several partial targets of the same or different partial output which is stable over a long period and is reproducible over the time of use of a target.
For depositing layers on substrates having a diameter in excess of 100 mm--with layer thicknesses and layer properties of very good homogeneity--it is also known to separate the target into several partial targets. In such a magnetron sputtering source, also known as a dual ring source, two partial targets are concentrically arranged with a discharge plasma burning in front of each partial target (DE 4,127,262 C1). The output density and, hence, the target erosion on the concentric partial targets differ from each other by a factor of 2 to 5, depending on the coating geometry. Therefore, the change of the plasma parameters differs significantly with increasing target erosion. In a non-reactive operation, it is possible and sufficient to set the uniformity of the coating thickness by selection of partial discharge outputs. A great disadvantage of the reactive operating mode of such a magnetron sputtering source resides in the excessively high demands put on the process to obtain a homogeneity of the stoichiometry which is stable over a long time and which is reproducible. The reasons for this are that the stability of the discharge, its operating point and, therefore, its stoichiometry are decisively determined by the ratio of output density to the supply of reactive gas. Some layer properties (e.g., the hardness) and the rate of deposit depend, aside from the stoichiometry, also upon the set values of the discharge voltage and/or the discharge current {P. Frach et al., Surface and Coating Technology, 59(1993) 177]. If only the partial discharge output is stabilized, it will in the end result in a drift of the coating properties and its homogeneity. Thus, for example, a brief localized variation in the flow of oxygen in the reactive sputtering of Al.sub.2 O.sub.3, from the Al target, leads to a shift of the operating point of the discharge of the given partial target and, hence, to changed properties.
Furthermore, it is known to carry out reactive magnetron discharge by applying a pulsed voltage (alternating voltage) to two targets of different materials. The process parameters of discharge current and voltage and the gas pressure are measured by sensors and compared with a desired value. To obtain identical sputtering velocities of both targets, the signals obtained are processed in a control program to yield qualitatively good layer properties on the substrate (DE 4,324,683). The process suffers, however, from the fact that it is either not possible or difficult to control the discharge output or output density of the individual targets or partial targets independently of each other. The reason for this is that the required parameters for comparison with the desired value cannot be detected for each target or partial target.